Deformable mirror

ABSTRACT

This invention relates to a hybrid deformable mirror. A deformable mirror is provided comprising a reflective surface provided on a substrate and a layer of deformable material attached to the substrate that is operable to deform the mirror and wherein the substrate is supported by an actuator that is operable to deform the mirror.

This application is the U.S. national phase of international applicationPCT/GB2003/005555, filed 18 Dec. 2003, which designated the U.S. andclaims priority of GB 0230038.2, filed 23 Dec. 2002, and GB 0309976.9,filed 30 Apr. 2003, the entire contents of each of which are herebyincorporated by reference.

This invention relates to a deformable mirror. In particular, thisinvention relates to a hybrid deformable mirror and is particularlywell-suited to large deformable mirrors that may be used in applicationssuch as astronomical telescopes and high resolution imaging.

Deformable mirrors are often used in the field of adaptive optics. Forexample, phase distortions in a signal may be sensed by a wavefrontsensor and these distortions may be corrected for by deforming anadaptive mirror. Such adaptive mirrors may be employed in numerousfields, including:

-   -   imaging, for example adaptive mirrors are used in astronomy to        improve the resolution of earth-based telescopes that are        otherwise affected by atmospheric distortions;    -   laser sensing, where the amount of laser light that can be        delivered onto a target is significantly increased by using an        adaptive mirror to correct for atmospheric distortions—this        enables either better information to be obtained or objects to        be identified at a greater range; and    -   laser generation, where an adaptive mirror can be used        intracavity within a high power laser to counter the thermal        blooming that can be otherwise induced by the high concentration        of laser light inside the cavity.

Phase distortions can be corrected conveniently by reducing thedistortions into characteristic Zernike modes. Zernike created apolynomial power series that provides a mathematically convenient way todescribe the phase of an optical beam. Each term of the expansionincludes a coefficient multiplied by a mathematical expression whichrepresents a potential form of aberration, e.g. focus, coma orastigmatism. Increasing terms or modes are increasingly complex. Forexample, the first two Zernike modes are associated with tip-tilt. Thesemodes can be filtered off and processed to provide the control signalsfor a separate tip-tilt mirror. The third Zernike mode often relates tofocus. The focus and other higher order modes can then be processed toprovide control signals for the deformable mirror.

To date, deformable mirrors use one or other of two alternativedeformation mechanisms. The first is a class of mirrors called zonalmirrors. This class of mirrors is based on a number of discretepiezoelectric actuators attached directly to a deformable mirror. Eachactuator in a zonal mirror can be used to deform an area of the mirrordirectly above it.

The second is a class of mirrors that generally comprise a substratebonded to an active element. The active element is controlled such thatthe mirror is made to deform to adopt a desired shape, for example aconvex shape, and this in turn causes the substrate to bend to the sameshape. The active element is usually a piezoelectric material bonded toa substrate using an epoxy resin. The mirror can either have a singlelayer of piezoelectric material bonded to the substrate (strictlyspeaking a unimorph), or can be a dual piezoelectric layer with the twopieces poled in opposite directions (this is a true bimorph).

For smaller mirrors, bonded piezoelectric elements (e.g bimorphdeformable mirrors) are preferred due to their relatively low cost. Suchmirrors provide an adequate balance between bandwidth and stroke.However, the balance between bandwidth and stroke is especiallyimportant when looking to make larger mirrors e.g. mirrors with activeapertures greater than 10 to 15 cms. In order to keep the resonantfrequency and thus the bandwidth of the mirror constant, the thicknessof the substrate must also increase. For the larger mirrors this willadversely affect the minimum curvature available from the mirror. Forthis reason, larger mirrors have historically been zonal mirrors. Thefact that the substrate is supported by a large number of actuatorsmeans that the resonant frequency, and therefore bandwidth, is no longerdirectly linked to the mirror diameter. However, the overriding issuewith this type of deformable mirror is the cost. Although there are anumber of different actuator technologies available, none of them arecheap. This makes large mirrors expensive because as many as 300actuators may be required. For a bonded piezoelectric element mirror,although a large peizoelectric element will be more expensive than asmaller one, the cost differential will not be as great. A second issueis that it is not always possible to place discrete actuators as closeto each other as required because of their fairly large size.

Against this background, and from a first aspect, the present inventionresides in a deformable mirror comprising: a passive substrate layerhaving a reflective surface provided thereon; a first layer of activelydeformable material, attached to the passive substrate layer, that isoperable to deform the mirror as a result of transverse expansion orcontraction of the material under the influence of a field appliedacross its thickness; and an actuator coupled to one of said layers thatis operable to further deform the mirror. The actuator can be used toprovide the basic deformation required of the mirror (e.g. focus), whilethe deformable material can be used to provide fine tuning of the mirrorshape. In this arrangement, the substrate no longer needs to besupported from the edge and so the resonance frequency and bandwidth isincreased over and above what it would be for a purely edge-supporteddevice. This means it is possible to concentrate on optimising thedesign of the deformable material to give the maximum curvature withless constraint from the resonance effects.

Preferably, the deformable mirror comprises a plurality of actuatorsthat support the substrate. Optionally, the actuators are arranged to beoperable to correct lower order Zernike modes. Preferably, the layer ofdeformable material is segmented, the segments being arranged to beoperable to correct higher order Zernike modes. Optionally, thedeformable material comprises peizoelectric material. Preferably, theactuator comprises magnetostrictive or electrostrictive material.

From a second aspect, the invention resides in a method of correctingphase variations in a beam of electromagnetic radiation incident upon adeformable mirror described above, wherein the actuator or actuators aremoved to correct Zernike modes at or below a threshold order and thefirst and/or second layer or layers of actively deformable materialis/are moved to correct Zernike modes above the threshold order. Otherpreferred, but optional, features of the invention are set out in theappended claims.

In order that the invention can be more readily understood, referencewill now be made, by way of example only, to the accompanying drawingsin which:

FIG. 1 is a cross-sectional view through the centre of a deformablemirror according to a first embodiment of the present invention;

FIG. 2 is a cross-sectional view through the centre of a deformablemirror according to a second embodiment of the present invention;

FIG. 3 is a cross-sectional view through the centre of a deformablemirror according to a third embodiment of the present invention;

FIG. 4 is a cross-sectional view through the centre of a deformablemirror according to a fourth embodiment of the present invention;

FIG. 5 is a cross-sectional view through the centre of a deformablemirror according to a fifth embodiment of the present invention;

FIG. 6 is a cross-sectional view through the centre of a deformablemirror akin to that of FIG. 5;

FIG. 7 a shows in plan-form the arrangements of actuators on thedeformable mirror of FIG. 6;

FIG. 7 b shows in plan-form an alternative arrangement of actuators on adeformable mirror;

FIG. 8 is a plan view of a deformable mirror and a mount according to asixth embodiment the present invention;

FIG. 9 is a cross-section through line IX-IX of FIG. 8 showing themirror in a relaxed state;

FIG. 10 corresponds to FIG. 9 but with the mirror in a state ofdeformation;

FIG. 11 is a detail from FIG. 9;

FIG. 12 is a perspective view of part of the mount of FIG. 9;

FIG. 13 corresponds to FIG. 11 but for a seventh embodiment of thepresent invention;

FIG. 14 corresponds to FIG. 11 but for a eighth embodiment of thepresent invention;

FIG. 15 is a cross-sectional view of a ninth embodiment of the presentinvention;

FIG. 16 is a perspective view of part of the mount of FIG. 15, with themirror removed;

FIG. 17 is a further perspective view of a part of the mount of FIG. 15,with the mirror in place.

In all embodiments described herein, the mirror 10 comprises a coppersubstrate 14 whose outer face 16 provides a reflecting surface by virtueof a series of thin dielectric coatings provided on the outer surface 16(not shown). At least one active piezoelectric element 18 is bonded tothe substrate 14 using epoxy resin 20. An array of electrodes 22 areused to activate the piezoelectric element(s) 18. Applying a potentialto the electrodes 22 causes the piezoelectric element(s) 18 to deform sothat, in turn, the substrate 14 deforms to create a mirror 10 with adesired shape, convex for example. In order to enable the stress inducedby the piezoelectric element 18 to be coupled effectively to thesubstrate 14, the epoxy resin 20 should be as thin as possible. However,if the resin 20 is to be very thin, it must also be uniform. If theepoxy resin 20 is too thick there will be unnecessary loss of couplingefficiency; if the epoxy resin 20 is too thin, the sheer strength of theglue will be compromised. This is of course true for all embodiments ofthe present invention illustrated herein. In a further embodiment of theinvention, glass spacers (normally used in the manufacture of liquidcrystal devices) are interposed between the substrate 14 and thepiezoelectric element 18 thereby enabling a uniformly thin layer ofepoxy resin 20 to be achieved between the substrate 14 and thepiezoelectric element 18.

In addition, each embodiment includes at least one actuator fabricated24 from magnetostrictive or electrostrictive material. The actuators 24are attached at one end to a base 25 of a mount 26 and, at its otherend, either to the piezo-electric element 18 or directly to thesubstrate 14. Applying a potential to the actuator 24 causes it toexpand or contract thereby deforming the substrate 14 and hence themirror 10.

Turning first to the embodiment of FIG. 1, the mirror 10 is firmlysecured around its periphery to a mount 26. The mirror 10 is disc-shapedand the mount 26 is cylindrical with a circular aperture in the centreof its top, the aperture being sized and shaped to receive the mirror10. The internal sides 28 of the mount 26 are stepped so as to provide ashoulder 30 for seating the mirror 10.

In this first embodiment, a single actuator 24 is rigidly bonded to boththe base 25 and the centre of the piezoelectrical element 18. Theactuator 24 is in the shape of an elongate cylinder and is stepped toform a narrowed head 32 at its top. In this particular embodiment, onlythe Zernike mode for focus needs to be filtered out and so a singleactuator 24 placed at the centre of the mirror 10 is sufficient to meetthis requirement.

Further embodiments of the present invention will now be described. Theembodiments are very similar and so corresponding parts have beenassigned corresponding reference numerals and will not be describedagain in order to avoid unnecessary repetition.

A second embodiment of the present invention is shown in FIG. 2. Incommon with the embodiment of FIG. 1, this second embodiment has amirror 10 firmly secured around its periphery to a mount 26, and alsohas a central actuator 24 bonded between the mirror substrate 14 and themount's base 25. Moreover, the second embodiment differs in that it hasa further six actuators 24 (of corresponding design to the centralactuator 24) arranged concentrically halfway along six equispaced radiiof the mirror 10. Assuming that the first two Zernike modes are alreadyfiltered off to provide the control for a tip-tilt mirror, the sevenactuators 24 are used to correct for distortions associated with thenext few Zernike modes, whilst the piezoelectric element 18 is used tocorrect for distortions associated with all the remaining higher orderZernike modes.

FIG. 3 shows a third embodiment of the present invention in which thereare a larger number of actuators 24 supporting the mirror. With thismany actuators 24, there is no longer any need to support the mirror 10round its edge. With the edge of the mirror 10 free to move, there willbe fewer constraints on the mirror deformation around its edge. However,the piezoelectric element 18 can provide fine control of mirrordeformation, and this helps to keep the number of actuators 24 requiredto a minimum (this minimum will be dictated by the bandwidth that isrequired from the mirror 10).

In the example shown there are a total of thirteen actuators 24 arrangedas follows: there is a single central actuator 24 surrounded by twoconcentric rings of six actuators 24. The rings are placed one third andtwo thirds of the way along the radius of the mirror 10 and theactuators 24 within the rings are arranged such that they are paired tosit on a common radius. FIG. 4 shows a further embodiment of the presentinvention that is very similar to the third embodiment. It differs inthat rather than having a single piezoelectric element 18 to which theactuators 24 are bonded, the piezoelectric material 18 is segmented intoan array of piezoelectric elements 18. An advantage of this approach isthat the overall size of the mirror 10 is not limited by the size ofmonolithic piezoelectric element 18 available. Again, the individualelements 18 can be made of either a single piezoelectric layer, abimorph structure or multilayer elements. However, in order to take fulladvantage of a multilayer piezoelectric element, contact should be madeto the electrode 22 in each layer. Furthermore, gaps are left betweenpiezoelectric elements 18 so that the actuators 24 can be attacheddirectly to the mirror's substrate 14. The advantage of this arrangementis that the actuators 24 can be screwed into the back of the mirror'ssubstrate 14, thereby providing a very strong attachment.

Actuators 24 can be screwed directly into the substrate 14 even where amonolithic piezoelectric element 18 is used, as follows. Holes arepunched through the monolithic piezoelectric element 18 using anultrasonic drill where the actuator 24 needs to pass through.

In a fifth embodiment shown in FIG. 5, twelve actuators 24 are used tosupport the mirror 10 around its edge. In this embodiment, apiezoelectric element 18 is used to provide the deformations requiredfor the main part of the mirror 10, while the actuators 24 are used toprovide the correct boundary conditions at the edge of the mirror 10. Analternative, but broadly similar, arrangement is shown in FIGS. 7 a.This arrangement differs only in that a central actuator 24 a has beenadded in addition to twelve actuators 24 b arranged around the edge ofthe mirror 10. The outer actuators 24 b may be used to provide thecorrect boundary conditions for the mirror 10, whilst the centralactuator 24 a may be used to correct for focus (the third Zernike mode).All higher Zernike modes can be corrected using the piezoelectricelement 18. FIG. 7 b shows a slightly modified arrangement: anintermediate ring of six actuators 24 c have been added between thecentral actuator 24 a and the twelve outer actuators 24 b. Theintermediate actuators 24 c may be used with the central actuator 24 afor control of low order distortions.

A deformable bimorph mirror 50 and a mount 52 according to a sixthembodiment of the present invention are shown in FIGS. 8 to 12. Themount 52 is a unitary structure made from stainless steel. The mount 52comprises a round body 54 that defines a central circular aperture 56.The aperture 56 is shaped and sized to receive the disc-shapeddeformable bimorph mirror 50 therein. Hence, the mirror 50 is held in aprotected position within the mount 52.

Whilst the outer edges of the mount's body 54 are regular, the internaledges 58 are stepped to form a series of three interconnected andconcentric circular apertures 56 a-c that increase in size from top tobottom. The stepped inner profile 58 of the mount 52 produces a seriesof three shoulders 60 a-c. Twenty generally L-shaped flexible beams 62extend downwardly in cantilever fashion from the topmost 60 a of theseshoulders 60 a-c. The twenty beams 62 are of identical size and shapeand are equispaced around the circular topmost shoulder 60 a. The beams62 are L-shaped such that they extend downwardly from the topmostshoulder 60 a before turning through 90° to extend inwardly towards thecentre of the middle aperture 56 b. Rather than having a pure L-shape, asquare-shaped support shoulder 64 extends from the internal corner ofeach beam 62 as best seen in FIG. 11. The support shoulder 64 onlyextends partially up the height of the upright portion 66 of the beam62, thereby leaving a narrow neck 68 in the portion of the beam 62 thatbridges the topmost shoulder 60 a of the mount body 54 and the supportshoulder 64 of the beam 62. It is this neck 68 that gives the beam 62its flexibility, i.e. this neck 68 can be deformed to allow the beam 62to deflect and bend. The length and thickness of the neck 68 of thebeams 62 are chosen to achieve the desired flexing properties. FIG. 12shows four of the beams 62 in perspective and indicates the width W ofthe beams 62 relative to their separation. It is the relative width ofthe beams 62 that gives the required degree of stiffness in the plane ofthe mirror 10.

The inwardly-extending portion 70 of the beam 62 extends beyond thesupport shoulder 64 to provide an upwardly-facing support surface 72 forreceiving the mirror 50. The mount 52 and the beams 62 are sized suchthat the mirror 10 may be received within the beams 62 to be supportedfrom below by the support surfaces 72 and so that the mirror's edge 74fits snugly against the upright face 76 of the support shoulders 64.Hence, the mirror 50 is held firmly in place.

The mirror 50 is best seen in FIG. 9 and corresponds to the mirrorsdescribed previously. To recap, the mirror 50 comprises a coppersubstrate 78 whose outer face provides a reflecting surface by virtue ofa series of thin dielectric coatings provided on the outer surface (notshown). An active piezoelectric element 80 is bonded to thenon-reflective side copper substrate 78 using epoxy resin 82. An arrayof forty-five electrodes 84 are used to activate the piezoelectricelement 80. The mirror 50 is also supported from below by an array ofseven magnetostrictive actuators 24 that extend between a base cap ofthe mount 52 and a lower surface of the mirror 50 where they attach asdescribed previously. Applying a potential to the electrodes 84 and theactuators 24 cause the piezoelectric element 80 and the actuators 24 todeform so that, in turn, the copper substrate 78 deforms, as shown inFIG. 10. This creates a convex-shaped mirror 50.

As the mirror 50 deforms, it remains firmly held in place against thesupport surface 72 and support shoulder 64 because the beam 62 deflectswith the mirror 10 by flexing about its neck 68, as shown in FIG. 10.Moreover, the beams 72 offer minimal resistance to the mirror 50 as itsperipheral edge 74 rotates towards the mirror axis. This is because theyhave minimal stiffness radially and so require little force to deformradially towards the mirror centre. The mass and stiffness of the beams62 are very small in comparison to that of the mirror 50 and thereforethe beams 62 have minimal impact upon the mirror 50 deformation. Inaddition, the relatively large width W of the beams 62 providesstiffness in all directions in the plane of the mirror and torsionallyabout the mirror axis. The short length of the beams 62 providesstiffness in the axial direction.

FIG. 10 shows that convex deformation of the mirror 50 extends to thevery edge 74 of the mirror 50 and hence eliminates virtually all deadspace from the mirror 50. Hence, the active area of the mirror 50 coversvirtually the whole of the mirror 50. This is highly beneficial becausea mirror mount 52 that prevents rotation of the mirror's peripheral edge74 would need to be twice the diameter to obtain a similar convex activearea and would have a first mode resonant frequency of half that of thesimply supported mirror 50 of the present invention. Thus, the presentinvention allows for a mirror 50 of much smaller size to be used toobtain the same stroke/bandwidth product.

The person skilled in the art will appreciate that modifications can bemade to the embodiments described hereinabove without departing from thescope of the invention.

Use of unimorph, bimorph or multilayer piezoelectric elements 18 can befreely varied according to need in any of the embodiments. In addition,the type of actuators 24 used may be varied in each embodiment. Suitabletypes of actuators 24 include magnetostrictive, electrostrictive,piezoelectric, electromagnetic, hydraulic, mechanical orelectromechanical. An example of an electrostrictive material is PMN(lead magnesium nitrate).

The arrangement of actuators 24 given herein are merely examples ofpreferred configurations and may be varied without departing from thescope of the invention. Moreover, many types of standard configurationsof piezoelectric elements 18; 80 can be used in order to obtain thedesired deformation of the mirror 10; 50. In particular, the choice ofusing a monolithic piezoelectric element 18; 80 or an array of discretepiezoelectric elements 18; 80 can be made for each of the embodimentsshown.

Details of the mirror 10; 50 and how it is arranged to deform are givenas useful background in which to set the context of the presentinvention, but are not essential to the invention. Other mirrorconfigurations can be equally well accommodated by the presentinvention.

Whilst one of the above embodiments uses L-shaped beams 62, strictcompliance with this shape is not necessary. For example, the supportshoulders 64 may be omitted and the peripheral edge of the mirror 74 mayabut against the upright face of the beam 62. This arrangement wouldlead to a longer neck 68 that could flex along its entire height. Inaddition, the beam 62 could be J-shaped rather than being L-shaped. Thismay be advantageous where the mirror 50 has rounded edges rather thansquare edges. In fact, the beam 62 may be shaped to conform to anyprofile the mirror 50 may have, e.g. to conform to chamfered edges.

Furthermore, the beams 62 need not necessarily extend downwardly fromthe mount body 54 to house the mirror 50 within the mount body 54. Analternative arrangement is shown in FIG. 13, that broadly corresponds tothe view shown in FIG. 11 and so like reference numerals have been usedfor like parts but with the addition of a prime. In this embodiment, theflexible neck 68′ is L-shaped such that, in addition to the flexibleupright portion 86′ that allows deflection as the mirror 50 deforms,there is a horizontal portion 88′ that connects the upright portion 86′to the mount body 54. The horizontal portion 88′ of the beam 62′ allowsvertical movement of the edges of the mirror 50, as indicated by thearrows in FIG. 13. This is beneficial because the mirror 50 may bedeformed to adopt shapes that require relative movement around the edge74 of the mirror 50, e.g. to adopt radially-extending ridges and troughsthereby creating an undulating mirror edge 74.

A further alternative arrangement of the beams 62 is shown in FIG. 14where beams 62″ extend upwardly from the mount body 54′ (like referencenumerals are used for like parts, the double prime denoting the partsthat belong to the embodiment of FIG. 14). Most importantly they retainthe flexible neck 68″ that allows the beam 62″ to bend with the mirror(not shown) as it adopts a convex shape.

A yet further embodiment is shown in FIGS. 15 to 17. Again, likereference numerals are used for like parts, the triple prime denotingthe parts that belong to the embodiment of FIGS. 15 to 17. In thisembodiment, the mirror 50′″ is supported at the top of the mount 52′″.The mount 52′″ has an outer wall 90′″ extending from the outer edge ofits top surface. Twenty flexible beams 62′″ extend from the inner edge60 a′″ of the mount 52′″. The flexible beams 62′″ comprise an L-shapedflexible neck 68′″ that extends from the mount 52′″ first upwardly as anupright portion 86′″ before turning through 90° to extend inwardly as ahorizontal portion 88′″. The horizontal portion 88′″ of each of theflexible beams 62′″ meets a unitary L-shaped annular ring 92′″ that isshaped and sized to receive the mirror 50′″. The L-shape of the ring92′″ is such that it supports the mirror 50′″ from the side and frombelow.

The advantage of this arrangement is that the shape of the flexiblebeams 62′″ allows vertical movement of the mirror's edge. This providesadditional enhancement by further minimising the ratio of the totaldiameter of the mirror 50′″ to the active diameter. This reduces theoverall mirror diameter required to achieve a given stroke for a setapplied voltage and bandwidth by virtually eliminating any dead spacefrom the outside of the mirror 50′″.

As will be appreciated by the skilled person, other arrangements of thebeams 62 are possible. For example, the flexible beams 62 could extendinwardly to meet a supporting end of the beam 62. Essentially, anyarrangement could be used where the supporting end of the beam 62 isconnected to the mount body 54 by a flexible neck 68 that allows thesupporting end to bend as the mirror 50 deforms.

Whilst the mount 52 of the above embodiments is made from stainlesssteel, many other materials such as other metals, plastics, glasses orceramics could be used instead.

1. A deformable mirror comprising: a passive substrate layer having areflective surface provided thereon; a first layer of activelydeformable material, said first layer having a thickness and attached tothe passive substrate layer for deforming the mirror as a result oftransverse expansion or contraction of the deformable material under theinfluence of a field applied across said thickness; and a linearactuator coupled to one of said layers for further deforming the mirror.2. A deformable mirror according to claim 1, wherein the first layer ofactively deformable material is bonded to the passive substrate layer.3. A deformable mirror according to claim 1, comprising a second layerof actively deformable material bonded to the first layer of activelydeformable material.
 4. A deformable mirror according to claim 1,comprising a plurality of linear actuators, each of said actuatorscoupled to one of said layers.
 5. A deformable mirror according to claim4, wherein the linear actuators correct lower order Zernike modes.
 6. Adeformable mirror according to claim 1, wherein the first layer ofactively deformable material is segmented and the segments are arrangedto correct higher order Zernike modes.
 7. A deformable mirror accordingto claim 1, wherein the first layer of actively deformable materialcomprise piezoelectric material and the field is an electric field.
 8. Adeformable mirror according to claim 1, wherein said linear actuator iscoupled directly to the passive substrate layer through one aperture inthe first layer of actively deformable material.
 9. A deformable mirroraccording to claim 1, wherein the linear actuator is comprise of one ofmagnetostrictive and electrostrictive material.
 10. A deformable mirrorholder for a deformable mirror according to claim 1, wherein the holdercomprises a body with a central aperture for receiving the deformablemirror, the central aperture being defined by a plurality of flexiblebeams, with each flexible beam having an end shaped to provide asupporting surface and a flexible portion that connects an end of thebeam to the body.
 11. A deformable mirror holder according to claim 10,wherein the ends of the flexible beams are co-joined to form a unitarystructure shaped to provide a supporting surface.
 12. A deformablemirror holder according to claim 10, wherein the ends of the beams liein the plane of the body of the holder such that, in use, the deformablemirror is received within the body of the holder.
 13. A deformablemirror holder according to claim 10, wherein at least one beam isgenerally L-shaped such that one leg of the L-shape provides theflexible portion and the other leg of the L-shape provides thesupporting surface of the end of the beam.
 14. A deformable mirrorholder according to claim 13, wherein the internal corner of theL-shaped beam has a shoulder that extends part of the way along bothlegs of the L-shape.
 15. A deformable mirror holder according to claim10, wherein the plurality of flexible beams are arranged around theentire aperture.
 16. A deformable mirror holder according to claim 15,wherein the width of the beams is larger than the separation betweenbeams.
 17. A deformable mirror holder according to claim 16, wherein thewidth of the beams is greater than four times the separation betweenbeams.
 18. A deformable mirror holder according to claim 13, wherein theperipheral edge of the deformable mirror is supported from below by oneleg of an L-shaped beam and is supported from the side by the other legof the L-shaped beam.
 19. A deformable mirror holder according to claim14, wherein the peripheral edge of the deformable mirror is supportedfrom below by one leg of the L-shaped beam and is supported from theside by an inwardly-facing side of the shoulder.
 20. A method ofcorrecting phase variations in a beam of electromagnetic radiationincident upon a deformable mirror according to claim 1, wherein theactuator is moved to correct Zernike modes at or below a threshold orderand the first layer of actively deformable material is moved to correctZernike modes above the threshold order.
 21. A method according to claim20, wherein the actuator is moved to correct the first and second orderZernike modes and the deformable material is moved to correct third andhigher order Zernike modes.
 22. A new deformable mirror according toclaim 3, wherein the at least one of said first and second layers issegmented and the segments are arranged to correct higher order Zernikenodes.
 23. A deformable mirror according to claim 22, wherein the linearactuator is coupled directly to the passive substrate layer by means ofat least one aperture in the first and second layers.
 24. A new methodof correcting phase variation in a beam of electromagnetic radiationincident upon a deformable mirror according to claim 3, wherein thelinear actuator is moved to correct Zernike modes at or below athreshold order and the layers of deformable material are arranged tocorrect Zernike modes above the threshold order.